The present applicant previously proposed an anti-cancer agent containing a phosphine transition metal complex represented by the following Formula (1′) (see PTL 1). In the formula, R1, R2, R3, and R4 are alkyl groups or the like, and M is gold, copper, or silver. This anti-cancer agent has an anti-cancer activity higher than that of an anti-cancer agent such as cisplatin or Taxol (registered trademark) that has been used in the related art.

The above phosphine transition metal complex contains 4 asymmetric phosphorous atoms, so there are numerous isomers for this complex. PTL 1 discloses that the type of the isomers is not particularly limited, and that the steric structure of the phosphorous atoms may be constituted with a single enantiomer such as (R,R)(R,R) or (S,S)(S,S) or with racemic isomers of ligands such as (R,R)(S,S). PTL 1 also discloses that the steric structure may be constituted with meso isomers such as (R,S)(S,R) or with one enantiomer and a meso isomer thereof such as (R,R)(S,R).